What آپ have to do is find a video. When آپ find the video that آپ want, آپ have to copy its ایل آر یو (Look below at image if آپ don't know what that is). When that's done, go to the فورم آپ want. When writing a new post, three square buttons should appear at the bottom: one is a smile, one is a camera and the other is a video camera. Click the video camera, and this rectangular box should appear. This is where your ویڈیوز ایل آر یو comes in. Paste the ایل آر یو inside the box (It usually doesn't read it if آپ just paste it, but hitting space afterwards usually does the trick). A picture of the video should appear below. If words can't do the trick, I made a small tutorial to help آپ out (Click the picture for a better resolution/to make it bigger).
